The invention relates to a method for controlling downlink beamforming in a radio communications system having an antenna device which comprises a number of antenna elements.
In radio communications systems, messages (speech, picture information or other data) are transmitted via transmission channels by electromagnetic waves (radio interface). The transmission process takes place both in the downlink direction from a base station to a subscriber station and in the uplink direction from the subscriber station to the base station.
Signals which are transmitted using electromagnetic waves are subject to disturbances from interference, inter alia, during their propagation in a propagation medium. Disturbances by noise may be caused, inter alia, by noise in the input stage of the receiver. Diffraction and reflections result in signal components passing through different propagation paths. Firstly, this means that a signal at the receiver is often a mixture of a number of contributions which, although they come from the same transmitted signal, they may, however, reach the receiver more than once, in each case from different directions, with different delays, attenuations and phase angles. Secondly, contributions to the received signals which are coherent but have different phase relationships may interfere with each other in the receiver, where they lead to cancellation effects over a short time scale (fast fading).
There are two classes of methods for combating transmission disturbances and/or interruptions caused by fast fading, by using antenna devices having a number of elements.
The first group is based on diversity techniques which, in simple terms, means that the downlink signal is transmitted at the same time on different channels, in the expectation that it will be possible to receive at least one of these channels at the subscriber station. Various diversity techniques are known, for example code diversity (code division transmission diversity (CDTD which is also referred to as orthogonal transmit diversity (OTD)), “time switched transmission diversity” (time switch transmission diversity TSTD) or selection transmission diversity (STD). These techniques are described, for example, in Raitola et al., Transmission Diversity in Whiteband CDMA, Proceedings of 49th IEEE Vehicular Technology Conference (VTC'99—Spring) Houston, Tex. Transmission using code diversity based on space time block codes as well as TSTD has also been mentioned in the specifications for the Third Generation Partnership Project for 3rd generation mobile radio networks, see 3GTS 25.211 Version 3.1.1 (Technical Specification Group Radio Access Network: Physical Channels and Mapping of Transport Channels onto Physical Channels (FDD), Version 3.1.0, December 1999).
Code diversity means that each antenna element in the antenna device transmits the same user data sequence, but in each case codes it using a different orthogonal code. In this case, in particular, the use of space time block codes for coding ensures that the contributions of different antenna elements to the downlink signal cannot cancel one another out at the receiver location. Self-cancellation of an individual contribution is, however, not precluded.
In TSTD, the downlink signal is in each case transmitted by different antenna elements in the antenna device at different times in accordance with a predetermined pattern.
These diversity techniques have the common feature that there is no feedback on the quality of the contribution of the individual antenna elements at the receiver, and that, therefore, the transmission power must be distributed virtually “blind” between the individual antenna elements at the transmitter. Thus, to a certain extent, these techniques follow a strategy of risk scattering: since the transmitter end does not know which of a number of available codes or which antenna element allows the best transmission at a given point in time, a number of codes or a number of antenna elements are used at the same time, or changed over rapidly, in the expectation that, on average, a usable transmission quality will be achieved, even if this is not the optimum.
In the case of selection transmission diversity (STD), as likewise described in Raitola and in other references, backward and forward switching between the transmitter antenna elements is carried out on the basis of feedback about the reception quality from the receiver to the transmitter. This technique makes it possible to specifically avoid the use of antenna elements which do not allow satisfactory transmission at a given point in time, and reduce the overall mean transmission power level of the transmitter.
If the transmission channel is changing only slowly, it is also possible for the receiver to determine weighting vectors, by which the contributions to the downlink signal transmitted by the individual antenna elements should be averaged at the transmitter end, in order to result in an optimum signal-to-disturbance ratio, and transmit these weighting values to the transmitter.
All these approaches have the common feature that they are feasible only for antenna devices with a maximum of two antenna elements. Specifically, if weighting vectors must be determined and must be transmitted to the transmitter, the bandwidth required to do this increases with the number of antenna elements; the bandwidth actually available for such a transmission is, however, tightly constrained. Control by weighting vectors that are transmitted in the opposite direction thus becomes more cumbersome the greater the number of antenna elements that are involved. In the case of diversity techniques such as OTD, TSTD, a diversity gain is admittedly achieved by using additional antenna elements; however, this gain is considerably less when changing from two to four antenna elements than when changing from one antenna element to two, that is to say the advantages which can be achieved by increasing the number of antenna elements are low in relationship to the complexity. Furthermore, these approaches do not offer any solution to the problem of the disturbance with individual receivers in a radio communications system resulting from downlink signals intended for other receivers.
One solution to this problem is achieved by beamforming methods. DE 198 03188 A1 may be cited as an example of a method such as this. This document discloses a method in which a three-dimensional covariance matrix is defined for a link from a base station to a subscriber station. An eigen vector is calculated from the covariance matrix in the base station, and is used as a beamforming vector for that link. The transmitted signals for that link are weighted with the beamforming vector, and are supplied to antenna elements for transmission.
Quite clearly, in an environment with multipath propagation, this method determines a propagation path with good transmission characteristics, and physically concentrates the transmission power of the base station on this propagation path.
However, this cannot prevent interference on this transmission path from briefly being able to lead to signal cancellation and thus to interruptions in the transmission.
With this method, directional transmission of the downlink signal makes it possible to considerably reduce disturbances with other receivers resulting from a downlink signal that is not intended for them. However, it is impossible to prevent interference on the directional propagation path from leading briefly to signal cancellation and thus to interruptions in the transmission. Furthermore, the method relies on the capability to associate a source direction with the uplink signal, in order to make it possible to transmit the downlink signal specifically in this source direction. However, this is not always possible. Especially in microcells, for example within buildings, multiple reflections may make it impossible to associate an uplink signal with one source direction. In an environment such as this, beamforming does not allow any better transmission qualities to be achieved than nondirectional transmission.